1. Field of the Invention
The present invention relates to a liquid crystal composition and a method for manufacturing the same, and in particular relates to manufacture a positive A film-embedded negative C optically anisotropic coating applied in a vertical alignment LCD.
2. Description of the Related Art
Phase compensator film or retardation film is used to solve large viewing angle problems for LCDs. The large viewing angle problems include reduction in contrast ratio, gray scale instability, gray scale inversion, color shift, and the likes. Conventional retardation films were made of polymer substrates formed by uniaxial stretching or biaxial stretching. However, with retardation value (R) equal to birefringence (Δn) times film thickness (d), because the birefringence of polymer materials is low, film thickness is required to be very thick to obtain appropriate retardation value. Thus, manufacturing yield of retardation films using polymer substrates are low as the conventional process requires costly machinery for stretching and control of the three dimensional refractive index (nx, ny, and nz) is difficult.
Normally, liquid crystal cell comprising rod-like molecules with positive birefringence can be counteracted by the optically anisotropic coating of retardation film with negative birefringence. For that, Fuji Photo Film, Ltd. (Japan) has disclosed a retardation film, prepared from disc-like liquid crystal molecules (named wide-view film), which is applied in TN or STN type LCDs. In U.S. Pat. No. 5,583,679, the described wide-view film was disclosed to be constituted by a three layered structure: a transparent polymer substrate bottom layer, a middle alignment layer for aligning the liquid crystal molecules, and an optically anisotropic top layer including hybrid aligned photopolymerizable disc-like liquid crystal molecules. Because disc-like liquid crystal molecules have inherently negative birefringence, the positive birefringence of the rod-like molecules in the liquid crystal cell can be counteracted. Furthermore, the wide-view film is formed by coating without additional stretching, therefore the thin retardation film is suitable for mass production by roll-to-roll manufacturing methods.
Recently, because various molecular alignments have been disclosed, mass producing optically anisotropic coatings of retardation film based on rod-like liquid crystal molecules are also developed. For example, a positive C (+C) optically anisotropic coating has rod-like liquid crystal molecules vertically aligned with a film surface, wherein the optically anisotropic coating has a refractive index of an x-axis similar to the refractive index of a y-axis, and the refractive index of the y-axis is less than the refractive index of the z-axis (nx˜ny<nz). A positive A (+A) optically anisotropic coating has rod-like liquid crystal molecules parallel aligned with a film surface, wherein the refractive index of an x-axis is greater than the refractive index of a y-axis, and the refractive index of the y-axis is similar to the refractive index of a z-axis (nx>ny˜nz). A cholesteric or negative C (−C) optically anisotropic coating has helical arranged rod-like liquid crystal molecules and the helical axis is vertically aligned with a film surface, wherein the optically anisotropic coating has a refractive index of an x-axis similar to the refractive index of a y-axis, and the refractive index of the y-axis is greater than the refractive index of a z-axis (nx˜ny>nz). An O optically anisotropic coating has rod-like liquid crystal molecules having a specific length axes tilt aligned with a film surface. A twisted nematic optically anisotropic coating has rod-like liquid crystal molecules arranged twist-like from 180° to 250°. A hybrid nematic optically anisotropic coating has rod-like liquid crystal molecules having a length axes tilt aligned with a film surface in continuous angles from 30° to 60°. The corresponding references disclosed that the optically anisotropic coatings can be used alone or in combination to compensate for viewing angle problems of different liquid crystal cells. For example, the twisted nematic optically anisotropic coating is applied to compensate for coloration problems and enhance contrast of STN-LCDs (see SID'95 Digest, 567 and SID'01 Digest, 452). Meanwhile, because hybrid nematic optically anisotropic coatings have similar symmetric molecular arrangements to the liquid crystal molecular arrangement of TN liquid crystal cells and ECB liquid crystal cell, integration of hybrid nematic optically anisotropic coatings may improve viewing angle properties (see SID'02 Digest, 762 and SID'05 Digest, 742). Additionally, because +C optically anisotropic coatings have rod-like liquid crystal molecules vertically aligned with a film surface, it can be applied in IPS-LCDs and circular polarized VA-LCDs (see SID'07 Digest, 1555).
In addition, cholesteric or negative C optically anisotropic coatings may improve viewing angles of VA-LCDs, because rod-like liquid crystal molecules in VA-LCD cells are vertically aligned in a non-driving state, its compensative effect is similar to +C optically anisotropic coatings due to its inherently positive retardation value. However, a typical 4 μm thick VA-LCD cell will over compensate and usually requires cholesteric or negative C optically anisotropic coating to counteract with a positive retardation value to obtain the best dark state. U.S. Pat. No. 6,141,075, WIPO Pat. No. WO 01/20393 and WO 01/20394 have disclosed that viewing angle compensation of VA-LCDs and TN-LCDs requires to collocate the positive and negative optically anisotropic coatings, wherein the negative optically anisotropic coatings should be −C optically anisotropic coatings. U.S. Pat. Nos. 6,141,075 and 6,281,956 also adopt a positive optically anisotropic coating collocated with a negative optically anisotropic coating to compensate for VA-LCD viewing angle problems. The described collocation of the positive and negative optically anisotropic coatings can be replaced by one or two pieces of biaxial stretching plastic plates such as triacetate cellulose (TAC) films. However, the stretched TAC film has very low birefringence, and it is necessary to use thicknesses of ten to hundred micrometers to obtain sufficient retardation value. Alternatively, side-stretching extrusion or casting of TAC films is possible, however, it is very difficult, and it is not easy to mass produce TAC films with acceptable yields. The WIPO Pat. No. WO 01/20393 disclosed that negative optically anisotropic coatings should be cholesteric type composed of UV photopolymerizable cholesteric liquid crystal (UV-CLC) molecules. Therein, UV-CLC molecules are spirally arranged with a short helical pitch (less than 200 nm) that is shorter than the wavelength of visible light. The patent also disclosed that the negative C optically anisotropic coating prepared from UV-CLC molecules had higher birefringence value and thinner thickness than a biaxial stretching plastic plate. In addition, the optically anisotropic coating can be mass produced by a roll-to-roll manufacturing methods.
A negative C optically anisotropic coating composed of pure UV-CLC molecules has a thickness of about 1 μm to 5 μm, such that retardation value depends on birefringence value (Δn). Conventional rod-like liquid crystal molecules have an Δn of about 0.1 to 0.16, and conventional disc-like liquid crystal molecules have an Δn of about 0.1 to 0.3. Meanwhile, the synthesis of photopolymerizable disc-like liquid crystal molecules requires time-consuming multi-steps, the disc-like alignment is difficult, and the preparation of room temperature nematic disc-like liquid crystals is difficult. As such, enhancement of retardation value is an important area of the UV-CLC formula. In addition, WO 01/20393 also disclosed that the viewing angle compensation of VA-LCDs and TN-LCDs require hybrid collocation of positive A and negative C optically anisotropic coatings to obtain a best dark state. However, conventional positive A and negative C optically anisotropic coatings are prepared in different steps and then adhered to top and bottom polarizer plates, respectively. Alternatively, positive A optically anisotropic coatings can be firstly prepared, and cholesteric liquid crystal molecules can be directly coated on the surface of the positive A optically anisotropic coating to form another negative C optically anisotropic coating. However, the bi-layered film from dual coating steps result in air gaps therebetween and non-uniformity. In addition, the upper negative C optically anisotropic coating is limited by the alignment ability of the lower positive A optically anisotropic coating, thereby resulting in formation of a foggy appearance and reduced transparency. Accordingly, adjusting the UV-CLC formula so that a single step coating is used to form a positive A optically anisotropic coating that directly contacts a negative C optically anisotropic coating is called for.